An electric power steering apparatus which provides a steering mechanism of a vehicle with a steering assist force (an assist force) by means of a rotational force of a motor, applies a driving force of the motor as the steering assist force to a steering shaft or a rack shaft by means of a transmission mechanism such as gears or a belt through the reduction mechanism. In order to accurately generate a torque as the steering assist force, such a conventional electric power steering apparatus (EPS) performs a feedback control of a motor current. The feedback control adjusts a voltage supplied to the motor so that a difference between a steering assist command value (a current command value) and a motor current detection value becomes small, and the adjustment of the voltage applied to the motor is generally performed by an adjustment of duty command values of a PWM (Pulse Width Modulation) control.
A general configuration of the conventional electric power steering apparatus will be described with reference to FIG. 1. As shown in FIG. 1, a column shaft (a steering shaft or a handle shaft) 2 connected to a steering wheel 1, is connected to steered wheels 8L and 8R through reduction gears 3 of the reduction mechanism, universal joints 4a and 4b, a rack and pinion mechanism 5, and tie rods 6a and 6b, further via hub units 7a and 7b. Further, the column shaft 2 is provided with a torque sensor 10 for detecting a steering torque of the steering wheel 1 and a steering angle sensor 14 for detecting a steering angle θ, and a motor 20 for assisting a steering force of the steering wheel 1 is connected to the column shaft 2 through the reduction gears (a gear ratio n) 3 of the reduction mechanism. Electric power is supplied to a control unit (ECU) 30 for controlling the electric power steering apparatus from a battery 13, and an ignition key signal is inputted into the control unit 30 through an ignition key 11. The control unit 30 calculates a current command value of an assist (steering assist) command on the basis of a steering torque Th detected by the torque sensor 10 and a vehicle speed Vel detected by a vehicle speed sensor 12, and controls a current supplied to the motor 20 on the basis of a voltage control command value Vref obtained by performing a compensation and so on with respect to the current command value. Moreover, the steering angle sensor 14 is optional and it may not be arranged.
A CAN (Controller Area Network) 50 for transmitting and receiving various kinds of information about the vehicle is connected to the control unit 30, and it is also possible to receive the vehicle speed Vel from the CAN 50. Further, a non-CAN 51 for transmitting and receiving communications, analog/digital signals, radio waves, etc. except for the CAN 50 is also connected to the control unit 30.
The control unit 30 mainly comprises a CPU (also including an MPU, an MCU, or the like), and general functions performed by programs within the CPU are shown in FIG. 2.
Functions and operations of the control unit 30 will be described with reference to FIG. 2. As shown in FIG. 2, the steering torque Th detected by the torque sensor 10 and the vehicle speed Vel detected by the vehicle speed sensor 12 (or from the CAN 50) are inputted into a current command value calculating section 31 for calculating a current command value Iref1. The current command value calculating section 31 calculates the current command value Iref1 that is a control target value of a current supplied to the motor 20 based on the steering torque Th being inputted and the vehicle speed Vel being inputted and by means of an assist map or the like. The current command value Iref1 is inputted into a current limiting section 33 through an adding section 32A. A current command value Irefm that a maximum current is limited, is inputted into a subtracting section 32B, and a deviation I (Irefm-Im) between the current command value Irefm and a motor current value Im being fed back, is calculated. The deviation I is inputted into a PI control (Proportional-Integral control) section 35 for characteristic improvement of steering operations. A voltage control command value Vref that the characteristic is improved by the PI control section 35, is inputted into a PWM control section 36. Furthermore, the motor 20 is PWM-driven through an inverter circuit 37 serving as a driving section. The current value Im of the motor 20 is detected by a motor current detector 38 and fed back to the subtracting section 32B. The inverter circuit 37 uses FETs as driving elements and is comprised of a bridge circuit of FETs.
A compensation signal CM from a compensation signal generating section 34 is added in the adding section 32A, and a characteristic compensation of a steering system is performed by the addition of the compensation signal CM so as to improve a convergence, an inertia characteristic and so on. The compensation signal generating section 34 adds a self-aligning torque (SAT) 343 and an inertia 342 in an adding section 344, further, adds the result of addition performed in the adding section 344 and a convergence 341 in an adding section 345, and then outputs the result of addition performed in the adding section 345 as the compensation signal CM.
As described above, such the electric power steering apparatus drive-controls the motor based on the current command value calculated on the basis of the steering torque inputted by a driver and the vehicle speed and provides the steering mechanism with the steering assist force generated by the motor through the reduction mechanism. Since the static friction exists in the steering mechanism due to friction of the motor itself, friction of the reduction mechanism, friction of a suspension and so on, the electric power steering apparatus is largely affected by the static friction of the steering mechanism.
Since the static friction of the steering mechanism is not taken into consideration in the conventional electric power steering apparatus, there is a problem that a feeling of being caught or the like due to the influence of this static friction of the steering mechanism occurs for example when the driver begins to steer, and the steering feeling in the vicinity of on-center (in the vicinity of a neutral position of the steering wheel), i.e. the on-center feeling gets worse.
In order to solve the above problem, for example, an electric power steering apparatus disclosed in Japanese Patent No. 3481468 (Patent Document 1) that improves the steering feeling by estimating static friction of the steering system and then compensating the estimated static friction, and an electric power steering apparatus disclosed in Japanese Patent No. 4852975 (Patent Document 2) that compensates a mechanical or electromagnetic loss torque so as to improve the on-center feeling of the steering wheel are proposed.
The electric power steering apparatus disclosed in Patent Document 1 calculates a static friction estimation value of the steering system based on the steering torque from the torque sensor and by means of a static friction calculating means, concretely, detects an increase in the steering torque caused by the static friction that occurs when the driver begins to steer and estimates the static friction of the steering system by focusing attention on a fact (a phenomenon) that “in the case that the static friction of the steering system is large even though being a steering angle by which the steering wheel (the handle) hardly moves, as a result, the steering torque increases, and in the case that the steering torque increases due to the influence of the static friction, a change in the steering torque is steep in comparison with after the motor begins to rotate and the steering angle begins to change”, and extracting a steering torque detection value of a part where the steering torque detection value shows a steep change.
Further, the electric power steering apparatus disclosed in Patent Document 2 calculates a friction weak field current which is a friction compensation value from a friction compensation calculation section and adds the weak field current to a d-axis current command value for controlling the strength of a magnetic field. In other words, the electric power steering apparatus disclosed in Patent Document 2 tends to be a configuration that compensates the loss torque with respect to a friction loss based on the structure of the motor and a loss based on an electromagnetic factor.